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髓鞘、兰式结和神经元内的跳跃传导. Sal Khan 创建




now that we know how a signal can spread through a neuron through an electro tonic potential and action potentials and combinations of the two let's put it all together by looking again at the structure of a neuron and anatomy of a neuron and thinking about why it has that Anatomy and how it all can work so we've already talked about the dendrites as being where the neuron can be can be stimulated from multiple inputs if we're in the brain this might be these dendrites might be near the terminal ends of axons of other neurons if we're some type of sensory cell this kind of these dendrites could be stimulated by some type of sensory input but let's just say for the sake of argument they are stimulated in some way and because they're stimulated in some way it allows it allows positive ions to flood in it allows positive ions to flood in to the neuron from the outside as we know there's a potential difference it's more negative in the neuron from the inside of the neuron than outside of the neuron and so if a channel gets opened up because of some stimulus that would allow positive ions that would allow positive ions to flow in and the primary positive ions we've been talking about are the sodium ions maybe this is some type of sodium gate that gets opened up because of this stimulus so when that happens you will have electrotonic spread you will have an electro electro tonic potential being spread so let's say that we had a voltmeter right here on the axon hillock it's kind of the hill that leads to the axon right over here so what you might see happening after some amount of time so let me draw so let's say this is our voltage in millivolts across the membrane our voltage difference I should say this is the passage of time let's say the stimulus happens at time zero but right at time zero we haven't really noticed it with our voltmeter our voltage right across the membrane right over there is that that equilibrium negative 70 millivolts but after some small amount of time this electrotonic potential has gotten to this point because all of these positive charges are trying to get away from each other it's gotten to that point and you might see you might see a you might see a bump in the voltage the in the voltage difference I guess I should say this thing might go up so it might look something like that now that by itself might not be we might have gotten the voltage difference low enough I guess we could say or we might not have gotten the the the voltage inside of the member inside the cell posit enough in order to trigger the voltage gated ion channels and so maybe nothing happens maybe maybe this right over here this is negative 55 millivolts and so that's what you have to get the voltage up to the voltage difference up to in order to trigger the in order to trigger the ion channels the ion channels right over there so those are the sodium channels to get positive charge in here's the potassium channels to get the positive charge out the axon hillock has a ton of these because these are really there once they get triggered they can trigger an impulse that can then go down the entire axon and maybe stimulate other things maybe in the brain or else this or whatever else this neuron might be connected to so maybe that stimulus by itself didn't trigger it but let's say that there's another stimulus that happens right at the same time or around the same time and that happens and on its own that might have caused a similar type of that might have caused a similar type of bump right over here but when you add the two together and they're happening at the same time they're combined bumps their combined bumps are enough to trigger an action potential in the hillock or series of action potentials in the hillock and so then you really have essentially fired the neuron so now all sorts of positive charge gets flushed in flushed into the neuron and then purely through electrotonic spread you will have you will have this electrotonic potential spread down the axon now this is the interesting part because we could think a little bit about what is the best way for an axon to be designed in general if you're trying to transfer current the ideal thing to do is the thing that you're transferring the current down should conduct really well and you could or you could say it has low resistance low resistance but if you want it to be surrounded by an insulator you want it to be surrounded so if this was a cross-section you want it to be surrounded by an insulator that has high resistance high resistance and the reason is is because the reason is is because you don't want the electric you don't want the potential to you don't want the the the the potential to leak across across your across your membrane high high resistance right over here if you didn't have something higher resistance around it then your signal will actually transfer will actually go your current would actually go slower this is true if you're just dealing with electronics if you just had a bunch of copper wires on one side and you had some copper wires that were surrounded by a really good insulator a really good resistor for example plastic or rubber of some kind the current is actually going to have less energy loss less energy loss that's going to travel faster when it's surrounded by an insulator so you might say okay well gee the best thing to do would be to surround this entire axon with a good insulator and for the most part that is true it is surrounded by a good insulator that is what the myelin sheath is so let's say we wanted to surround this whole thing with just one big one big grouping of Schwann cells so one big myelin sheath which is a good insulator it does not conduct current well so this right over here is just one big myelin myelin sheath one big myelin sheath right over here now what's the problem with this well if this axon is really long and let's say you know you're a dinosaur or something and you're you're just trying to go up your neck and your neck is 20 feet long or even a human being we're you know we're reasonable size and you're going several feet or even well whatever you want to go a reasonable distance purely with electrotonic with electrotonic spread your signal remember it dissipates your signal is going to be really weak right over here you're going to have a week you're going to have a weak signal on the other end it might not be even strong enough to make anything interesting happening at these terminals which wouldn't be strong enough to make to trigger maybe other neurons or whatever else might needs to happen at this other end so then you say okay well then why don't we try to boost the signal well how would you boost the signal they say okay I like having this myelin sheath but why don't we put gaps in the myelin sheath every so often and then those gaps would allow the membrane to interface with the outside and in those areas we could put some voltage gated child channels that can release action potentials when to in order to essentially boost the signal and that's is exactly what the anatomy of a typical neuron is like so instead of just one big insulating sheath like this it would let me make some gaps here whoops I wanted to do that in black so actually let me just draw it like this let me just erase this so clear and let me clear this that's good enough and so what we could do is we could put gaps we could put gaps in it right over here where the axon the axonal membrane itself can can interface with its surroundings and of course we know we call those gaps the nodes of ranvier or Ranvir and I'm not really sure how to pronounce it so let me put those gaps in here so you put those gaps in here so these are the myelin sheath and this right over here is a node of ranvier node I'll just these are nodes of ranvier around VA and right in those little nodes right in those nodes right where the myelin sheath isn't we can put these voltage-gated channels we could put these voltage-gated channels to essentially boost the signal if we if the signal have to go electrotonic Li all the way over here to be very weak it's going to dissipate as it goes down but it could be just strong enough right at this point in order to triggered these voltage-gated channels in order to essentially boost the signal again in order to trigger an action potential boosts the signal and now the signal is boosted a little dissipate dissipate dissipate boost and a little boost right over here again and then it'll dissipate dissipate dissipate and boost dissipate dissipate boost and so by having this combination you want the myelin sheath you want the insulator in order to keep in order to keep the transmission of the current too fast in order to have minimal energy loss but you do need these these areas where where the myelin sheath isn't in order to boost the signal in order for this in order for the action potentials to get triggered and so your signal can keep being can look I guess keep being amplified if we wanted to talk in kind of electrical engineering speak and this type of conduction where the signal just keeps boosting and you know if you were to just kind of superficially observe it it looks like the signal is almost jumping it's it's it gets triggered here then gets triggered here then it gets triggered here then it gets triggered here and then it gets triggered here this is called saltatory conduction saltatory salta saltatory saltatory conduction saltatory conduction it comes from the latin word salt ra or salt i'm once again i don't know how to pronounce my latin isn't too good but it comes from the latin word for salt re which means to jump around or to hop around and that's because it looks like the signal is hopping around but that's not exactly what's happening the signal is traveling passively through it gets triggered here in the axon hillock then it then it travels passively through electrotonic spread and then it gets boosted and you have the myelin sheath around it to make sure it goes as fast as possible and you get very little loss of signal and then it gets boosted at the nodes of ranvier because it triggers it triggers these voltage-gated channels again that lure that triggers an action potential and then your signal gets boosted and then it dissipates boosted dissipates boosted dissipates boosted dissipates maybe it could even get boosted again and then it can trigger whatever else it has to rigor